Electrical machines, in particular large electrical machines such as those used in power stations, should be monitored continuously and/or analyzed from time to time, in order to identify fault states in good time and to avoid uncontrolled failures, thus ensuring smooth operation.
WO 00/69062 discloses a system for monitoring a machine, in which the system emits a warning signal when fault states occur. In this instance, the system comprises a plurality of sensors, which record various parameters. For example, the shaft ground current, the shaft voltage, the extent of the change in the shaft ground current and the extent of the change in the shaft voltage are measured. An evaluation system produces the warnings as a function of the change in the shaft ground current, the change in the shaft voltage and the average shaft ground current. This document furthermore discloses that these various measured values can be recorded as a function of time.
EP 0 391 181 discloses an arrangement for detection of turn shorts in the rotor winding of electrical machines. This arrangement can be used to record the state of an electrical machine over a relatively long time period, although individual measurements can also be carried out. In this case, the shaft voltage is recorded between the machine shaft and ground. The harmonics which occur in the shaft voltage are used as an evaluation criterion. When there are no turn shorts in the rotor, the distribution of the ampere turns on the rotor circumference is symmetrical, and the rotor field of the machine has only the odd-numbered harmonics. If a turns short now occurs, then the rotor field also contains the even-numbered harmonics, since this results in asymmetry in the circumferential distribution of the ampere turns.
A further method, which is known to a person skilled in the art, is to measure the DC voltage resistance of the rotor winding during operation. This method can be used, for example, to measure the temperature in the rotor winding. However, only the equivalent resistance with a DC voltage is determined in this case.